Enzymatic carboxyl methylation of proteins is a post translational modification which reduces negative changes and alters their secretory structure and properties. It is associated with exocytotic release. Enzyme activity is very high in the posterior pituitary gland, where it is associated with methylation of neurophysins, and in the brain. We have found at least three still unidentified proteins, in addition to neurophysins contained within axons of brain origin in the pituitary gland. Determination of enzyme activity in hypothalamic nuclei demonstrate high activity in the magnocellular nuclei associated with production of vasopressin, oxytocin and neurophysins.